Handover in a mobile telecommunication system occurs when a call has to be passed from one cell to another as the mobile terminal moves between the cells. In a traditional “hard” handover, the connection to the current cell is broken, and then the connection to the new cell is established. Since all cells in a CDMA system use the same frequency, it is possible to establish the connection to the new cell before leaving the current cell. This is known as “soft” handover. Soft handovers require less power, which reduces interference and increases capacity. A mobile terminal in soft handover may be connected to two or more base stations simultaneously. In the uplink direction, the connected base stations receive the signals from the mobile terminal and the signals are combined in the network (e.g. in the radio network controller (RNC)), e.g. by means of selection combining. In the downlink direction, the mobile terminal receives signals from the connected base stations. The signals are combined in the RAKE receiver of the mobile terminal, e.g. by using Maximum ratio combining.
Each cell has a set of neighbouring cells and when the mobile terminal is in soft handover, the mobile terminal, also referred to as User Equipment (UE), is required to monitor the neighbouring cells transmitting on the same frequency. The mobile terminal is hence required to perform measurements on the neighbouring cells. Examples of different types of radio interface measurements are:                Intra-frequency measurements: measurements on downlink physical channels at the same frequency as the active set.        Inter-frequency measurements: measurements on downlink physical channels at frequencies that differ from the frequency of the active set.        Inter-RAT measurements: measurements on downlink physical channels belonging to another radio access technology than the current radio access network in UMTS, e.g. GSM.        
The Active Set (AS) is defined as the set of base stations, also referred to as Node Bs, that the mobile terminal is simultaneously connected to. Cells which are not included in the active set, but are included in a cell info list (CELL INFO LIST), belong to the Monitored Set, and cells detected by the mobile terminal, which are neither in the CELL_INFO_LIST nor in the active set, belong to the Detected Set.
In many cases there are neighbouring cells that for different reasons are never included in the measurement order that is sent from the network to the mobile terminal. It is therefore possible for the mobile terminal to detect cells not being specified in the network (i.e. in the CELL_INFO_LIST), i.e. the detected set cells. Further, in release '99 of the 3GPP specification it is not mandatory for the mobile terminal to report the cell identity of the detected set cells even if the radio network has requested the mobile terminal to report that. It is only mandatory to report the scrambling code of the cell.
It is currently possible to order the mobile terminal to measure on maximum 32 cells per measurement purpose (e.g. intra and inter frequency and inter RAT). The intra frequency cells that are reported in the CELL_INFO_LIST to the mobile terminal are part of the above mentioned monitored set. If there is more than 32 neighbouring cells, the list is simply truncated and truncated cells will be part of the detected set together with all other cells that are not defined as neighbouring cells to the cells defined in the active set (AS).
Thus, it is desirable to be able to add the detected cells that are not already defined as neighbouring cells to the active set (AS).
When a mobile terminal send a measurement report of a detected set cell to the radio network, the radio network may add it to the AS only if it is defined as a neighbouring cell to any of the cells in the AS. A problem is that the cell identity may be unknown, it is therefore not possible to simply add the reported cell to the active set. According to release '99 of the 3GPP specification, it is not possible to require the mobile terminal to report the cell identity. The only way to identify the reported detected set cell if the cell identity is not received is the scrambling code, and it is therefore difficult for the radio network to know if it is possible to include the reported detected set cell in the AS as there can be several cells with the same scrambling code.